Benzoin type condensation is of interest, among other aspects, to: 1) in biochemistry as a model to form carbon-carbon bonds, 2) it is the classical example of specific catalysis, 3) the benzoin condensation shown in reaction 1 below is of organic chemistry relevance for it represents one of the first organic reactions whose mechanism was proposed by Arthur Lapworth. The mechanism is shown in scheme 1. The first step of the reaction is the nucleuphylic attack of the CN− ion to the C═O of the benzaldehyde to form a cyanohydrin, subsequently the cyanohydrin attacks, in a nucleophilic manner, another benzaldehyde molecule to form the corresponding benzoin.


In the above reaction benzaldehyde is used as the starting chemical, while the dimerization of 2-pyridinecarboxaldehyde in the presence of KCN produces very stable ethenediols, (C. A. Buehler, Chem. Rev. 1964, 64, 7). From the dimerization of 2-pyridinecarboxaldehyde the product 1,2-di(2-pyridyl)ethene-1,2-diol results, this reaction was mistakenly referred to as the pyridoin condensation in resemblance to the benzoin reaction type shown below as Reaction 2. 1,2-Di(2-pyridyl)ethene-1,2-diol produces orange crystals when equal volumes of 2-pyridinecarboxaldehyde and glacial acetic acid or KCN are stirred together for several hours.

The benzoin compound has the —COCHOH— group while the ethenediol compounds have the —(HO)C═C(OH)— group. Polymers with —(HO)C═C(OH)— parts have been prepared by polycondensation of pyridazine-2,3-dialdehyde, pyrazine-2,5-dialdehyde or from pyrimidine-4,6-dialdehyde catalyzed with KCN; the product obtained from these reactions is poly[di-1,2-(diazinilidene)ethene-1,2-diol], as shown in Reaction 3 below, (H. R. Wiley, U.S. Pat. No. 4,260,757).

The drawbacks of the reported procedure to obtain ethenediols are that they are obtained from aromatic aldehydes which, as is well known in organic chemistry, are easily oxidized and therefore have to be previously subjected to purification procedures such as distillation so that they could be used for these type of reactions. The formation of the polymers shown in reaction 3 is from aromatic dialdehydes which are compounds sensitive to air because they are easily oxidized and difficult to obtain for the series of steps involved in the process, furthermore in some cases they are expensive ought to be made in situ to avoid oxidation prior to dimerization. Also, the catalyst (KCN) and solvents must be removed after each reaction to obtain pure products.
From the reaction at high temperature between 2-pyridinecarboxaldehyde and 2-pyridinemethanol, without catalyst and solvent, the product is 2-hydroxy-1,2-(2-pyridyl)-1-ethanone(2), which is deemed to be unstable in solution. Subsequently, compound (2) treated with solvents such as cyclohexane or ethyl acetate produces 1,2-di(pyridine-2-il)etheno-1,2-diol(1) or 1,2-di(pyridine-2-yl)ethane-1,2-dione(3)(2,2′-pyridyl) M. J. Percino, V. M. Chapela, S. Romero, C. Rodriguez-Barbarin, F. J. Melendez-Bustamante Journal of Chemical Crystallography, vol 36(5), 303, (2006) as shown in Reaction 4 that follows.

In addition, in the reaction between 2-pyridinecarboxaldehyde with (6-methylpyridine-2-yl)methanol shown below as reaction 5, the main products obtained are keto-enol compounds: 2-hydroxy-1,2-bis(6-methyl-2-pyridyl)-1-ethanone and 2-hydroxy-1-(6-methyl-2-pyridyl)-2-(2-pyridyl)-1-ethanone. Subsequent treatment with solvent produces 1,2-bis(6-methylpyridine-2-yl)ethane-1,2-dione and 1-(pyridine-2-yl)-2-(6-methylpyridine-2-yl)ethane-1,2-dione and in a much lesser quantity 1,2-bis(6-methylpyridine-2-yl)ethene-1,2-diol (M. J. Percino, V. M. Chapela, O. Urzua, H. Toribio, C. Rodriguez-Barbarin Journal of Chemical Research, (2007), 187).

Reactions identified as 4 and 5, show several disadvantages due to the fact that from the reactions between aromatic aldehydes in the presence of different pyridinemethanol derivatives are produced products such as compound (2) reaction 4, and that by changing the solvent the expected corresponding low molecular weight ethenediols and α-diketones (3) are produced, aside other compounds that are also produced in some instances, this is, there is a mixture of products.
The process of the current invention, named Percino-Chapela has as one of its main novel features that starting from pyridinemethanol derivatives, the dimerization or coupling of pyridinic alcohols reaction is carried out avoiding oxidation as is the case when the starting materials are the corresponding aldehydes. It is a one step process to obtain compounds having the ethenediol group —(HO)—C═C—(OH). The products are orange or brown powders which may indicate a high electronic conjugation in their structure and are soluble in cyclohexane, methanol and DMSO.
The process of the current invention has as characteristic features the following: a) it is carried out in the absence or presence of some solvent, b) in the process of the current invention, the temperature may or may not be used as catalyst, c) in the process of the current invention the reaction may or may not be catalyzed by the presence of a catalyst (acid or base), d) the products may be obtained and separated in an easy way by precipitation, e) in the process of the current invention starting from pyridinic alcohols, ethenediols may be produced, in a single step reaction, f) pyridinemethanol derivatives used as starting chemicals do not oxidize as easily, their handling is not complex and their price is low, g) dimers, trimers, up to polymers compounds show high electronic conjugation or show charge transference that makes them colored compounds, stable at room temperature and atmospheric pressure, h) The products obtained through the process of the current invention, dimers, trimers, oligomers up to polymers are produced which are stable with outstanding properties that make them useful in the fields of electronics, optical and as inhibitors in polymerization and as antioxidants.